This invention relates to novel polymers containing sulfonic acid groups or sulfonic acid salts and the process for preparing said polymers.
Hydrocarbon polymers generally fall into two classes, thermoplastic and thermosetting resins. Thermoplastic resins may be readily worked by heating the polymer to its softening or melting point. They may then be processed by such deformation methods as vacuum forming, extrusion of a melt, compression molding, etc. The thermoset resins can generally not be reworked once they have hardened. In general, thermoset resins owe their unique properties to covalent crosslinks between polymer molecules. The crosslinks may be obtained by the interaction of various monomers such as styrene in the presence of small amounts of divinyl benzene or by the reaction of epoxy-type resins with polyamines.
Uncured elastomers such as natural rubber and butyl rubber are thermoplastic. They may, however, be crosslinked or vulcanized by the use of sulfur and accelerators which react with the carbons of the unsaturated bonds in the polymer molecule to form in effect a thermoset product which can no longer be fabricated or worked except by machining. The vulcanized polymers have found wide utility because of the significant improvements in physical properties brought about by crosslinking. Natural rubber, for example, may be crosslinked or vulcanized by the use of sulfur which reacts with the unsaturated carbon atoms in the polymer molecule to form a bridge between two molecules so that one polymer molecule is covalently bonded to the second molecule. If sufficient crosslinks of this type occur, all molecules are joined in a single giant molecule. Once extensively crosslinked, the polymer is intractable and insoluble and can no longer be fabricated except possibly by machining. It has, however, significantly improved physical properties. Thus, by vulcanizing rubber, tensile strength, elasticity, impact resistance, flexibility, thermomechanical stability and many other properties are either introduced or improved.
A third class of polymers has recently been developed which, although pseudocrosslinked, have a softening point or softening range of temperatures and may even be dissolved in various solvents. At normal use temperatures, these polymers behave similarly to crosslinked polymers. At elevated temperatures, however, they are readily deformed and worked in the same manner as thermoplastic resins. Such polymers are said to be physically crosslinked. Examples of such materials are the ionic hydrocarbon polymers (ionomers). These products owe their unique properties to the fact that crosslinking is accomplished by ionic, rather than covalent, bonding between molecules of the polymer. Typical of these ionic polymers are copolymers of ethylene and ethylenically unsaturated mono- or dicarboxylic acids which have been neutralized by metal salts. (See, for example, British Pat. No. 1,011,981 and U.S. Pat. No. 3,264,272.)
Sulfonic acid ionomers have been prepared by copolymerizing a styrene sulfonic acid with other monomers to form plastic polymers containing ionic crosslinks. (See, for example, U.S. Pat. No. 3,322,734 incorporated herein by reference.)
Methods of sulfonating polymers are well known to the art. For example, aromatic-containing polymers are sulfonated by a method described in U.S. Pat. No. 3,072,618, wherein a complex of a lower alkyl phosphate and SO.sub.3 is used as the sulfonating agent. These sulfonated aromatic polymers have generally been sulfonated to a sufficient extent to be water-soluble in the form of their alkali salts. Other aromatic-containing resins have been sulfonated, converted to their alkali metal salts and used as ion-exchange resins. Water-soluble polymers have been prepared by reacting the aromatic rings in styrene-butyl rubber graft polymers with SO.sub.3 to form a viscous sulfonation product; see, for example, Soviet Pat. No. 211,079.
Attempts have been made to sulfonate unsaturated polymers. Natural rubber has been sulfonated by complexing chlorosulfonic acid with ethers and esters and reacting the complex with the rubber in solution; see, for example, German Pat. Nos. 582,565; 550,243 and 572,980, wherein water-soluble products were obtained by sulfonating the rubber and preparing salts of acids with alkalis, alkaline earths, heavy metals and organic bases. The highly-sulfonated rubbers were found to be water-soluble per se.
Saturated polyolefins have similarly been sulfonated utilizing complexes of lower alkyl phosphorus compounds and SO.sub.3. (See, for example, U.S. Pat. No. 3,205,285, which teaches that dyeability of polypropylene may be improved by reacting polypropylene fibers with an SO.sub.3 complex.) The reaction of such treated fibers with alkali salts improves their dyeability.
More recently sulfonic acid ionomers of unsaturated elastomers have been prepared using the complexing agents described in U.S. Pat. No. 3,072,618. These elastomeric sulfonic acid ionomers exhibit improved tensile strength in the gum state. More particularly, the green strength of the polymer is substantially enhanced; see, for example, Belgian Pat. No. 71,861 and its U.S. counterpart, patent application Ser. No. 877,849, now U.S. Pat. No. 3,642,728, incorporated herein by reference.
Acyl sulfates are known to be effective sulfonating agents for aromatic compounds; see, for example, Gilbert, Sulfonation and Related Reactions, pp. 22-23, Interscience, New York (1965).